Re: [Xen-devel] [PATCH v4] xen/arm: Add a clock property

[Dirk Behme <dirk.behme@xxxxxxxxxxxx>]

On 20.07.2016 14:46, Geert Uytterhoeven wrote:
> Hi Julien,
>
> On Wed, Jul 20, 2016 at 2:10 PM, Julien Grall <julien.grall@xxxxxxx> wrote:
> > On 20/07/16 12:49, Geert Uytterhoeven wrote:
> > > On Wed, Jul 20, 2016 at 1:01 PM, Julien Grall <julien.grall@xxxxxxx>
> > > wrote:
> > > > On 20/07/16 10:43, Geert Uytterhoeven wrote:
> > > > > On Tue, Jul 12, 2016 at 9:46 AM, Dirk Behme <dirk.behme@xxxxxxxxxxxx>
> > > > > wrote:
> > > > > > Clocks described by this property are reserved for use by Xen, and the
> > > > > > OS
> > > > > > must not alter their state any way, such as disabling or gating a
> > > > > > clock,
> > > > > > or modifying its rate. Ensuring this may impose constraints on parent
> > > > > > clocks or other resources used by the clock tree.
> > > > > >
> > > > > > This property is used to proxy clocks for devices Xen has taken
> > > > > > ownership
> > > > > > of, such as UARTs, for which the associated clock controller(s) remain
> > > > > > under the control of Dom0.
> > > > > I'm not familiar with using XEN at all, but I'm a bit puzzled...
> > > > >
> > > > > Can't you just add a clocks property to the (virtual) serial device node
> > > > > in DT?
> > > > > Then the (virtual) serial device driver can get and enable the clock?
> > > > There is no DT node for the Xen console (hvc). The UART used by Xen will
> > > > be
> > > > completely removed from the Device tree.
> > > Why is it removed?
> > Because the device is used exclusively by Xen and DOM0 should not touch it
> > at all (IRQs and MMIOs are not mapped).
> IMHO then it's Xen's responsability to make sure not to disable the clock(s).
>
> Who removes the device node from the DT? If Xen, can't it just remember
> which clocks were present in removed device nodes?

Yes, we are trying this with "moving" the clocks of the removed nodes to 
the Xen hypervisor node:
https://lists.xen.org/archives/html/xen-devel/2016-06/msg02605.html

The question discussed here is how to deal with these clocks properly in 
Linux, then.
Best regards

Dirk

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